Urbanization in the watershed has had a profound impact on the hydrology of Strawberry Creek. A significant increase in the amount of impervious surface area in the watershed along with extensive channel alteration and confinement has drastically altered the creek's hydrologic regime. This has resulted in accelerated streambed degradation or incision and subsequent bank erosion as well as the destruction of aquatic habitat. Urbanization has also caused a deterioration in water quality.
The extensive amount of impervious surface area in. the watershed has had two major impacts. Less rainwater can infiltrate into the ground to recharge groundwater levels so the water table has dropped over time and consequently the baseflow of Strawberry Creek is quite low throughout most of the year. Secondly, an excessive amount of surface runoff occurs during storms resulting in a very short response or lag time (interval between precipitation and runoff) in both forks of the creek. Lag time is further reduced by the construction of storm sewers which increase the rate of runoff transmittal to the remaining open channels. The increased peak flows cause scouring and incision of the streambed, accelerated streambank erosion and pose a greater flooding potential.
Increased development in the Strawberry Creek watershed has led to the culverting and diversion of significant reaches of the creek to accommodate stormwater flows and additional construction. Figure 6 shows the culverted reaches of the creek drainage system. Two major bypass culverts carry South Fork drainage from Strawberry Canyon underneath the Haas Recreation Area, Memorial Stadium, Kleeberger Field, and Gayley Road. Both culverts empty out near the faculty clubs at the east end of the central campus where the South Fork reappears. The creek has also been extensively culverted at the LBL complex in the canyon and in North Berkeley's Northside district to handle stormwater flows. Culverting of these creek channels has destroyed their inherent ecological and aesthetic qualities. Culverting also promotes higher peak flood flows.
Strawberry Creek has become very "flashy" meaning the flood peaks are higher whereas the baseflow between storm events is lower than it would be under natural conditions. Most of the sediment moved by the creek occurs during high peak flows. This produces highly turbid water that interferes with recreational use and aesthetic enjoyment of Strawberry Creek. In extreme cases, sediment may degrade aquatic habitat or even smother benthic macroinvertebrates. On the other hand, reduced baseflow decreases available habitat which is also deleterious to aquatic life. In general, decreased streamflows also lower the assimilative capacity of the creek to dilute pollutants.
The equilibrium process of streambed incision or downcutting led to channelization and channel confinement of Strawberry Creek as long ago as 1907 in an attempt to stabilize the eroding banks. Channel confinement has had several major impacts. Scouring of the streambed is accelerated because both peak flow velocities and streamflows have increased and lateral channel confinement concentrates this energy downwards. This scouring effect obliterates many of the natural pool areas within the creek, resulting in a significant loss of aquatic habitat and destruction of winter cover from peak storm flows. In addition, streambanks downstream of channelized reaches are extremely vulnerable to accelerated undercutting and erosion. Extensive reaches of Strawberry Creek have been laterally confined (Figure 7).